688 POPULAR SCIENCE MONTHLY. 



show that they are descended from ancestral yellow forms by re- 

 taining vestiges of this color on the base of the petals, as in the 

 water-crowfoot. The pale yellow flowers of (Enothera laciniata, 

 of the cultivated Eibes aureum, and of Diervilla trifida in fading 

 change to rose or red, exhibiting a tendency to develop red colora- 

 tion. Aquilegia canadensis produces scarlet flowers, which are yel- 

 low inside and rarely all over. There are two other species in the 

 Northern flora which exhibit similar coloring, Lonicera semper- 

 virens and Spigelia marylandica, and the former is sometimes yel- 

 low throughout. Myosotis is at first pale yellow, and changes to 

 sky-blue. But the best illustration of the transition from yellow to 

 blue is exhibited by the violet family; the smallest and simplest 

 species is yellow, the most highly specialized is blue, and all the 

 intermediate stages are presented by Viola tricolor. 



Honey-guides are exceedingly rare among yellow flowers. Cas- 

 sia chamcecrista, which has nearly regular, showy yellow flowers, has 

 two or three petals with a purple spot at base, while four of the an- 

 thers are yellow and six purple. It is interesting to compare with 

 this flower the change of color presented by Arnebia. When the 

 flower opens, each lobe of the yellow corolla is marked by a dark 

 purple spot, which soon begins to fade, and by the next day has en- 

 tirely disappeared. Saxifraga aizoides has golden flowers spotted 

 with orange, and attracts a large number of insect visitors, and the 

 yellow violets have their petals marked with dark-brown lines lead- 

 ing to the honey glands. Sulphur-yellow flowers are visited chiefly 

 by bumblebees, and their coloration seems to have been developed 

 by their selective influence from red or purple-flowered ancestors. 

 Miiller observed that the sulphur-yellow flowers of Sempervivum 

 Wulfenii, which are unlike the primitive yellow of the Crassulacece, 

 are purple at base. This purple coloring he believed to be a rem- 

 nant inherited from an earlier purple-flowered form. Hibiscus 

 trionum, which is sulphur-yellow with a blackish eye, has perhaps 

 been derived from a red-flowered ancestor, for the three other spe- 

 cies of the genus are rose or flesh colored. 



White flowers, in the opinion of the writer, are due to retrogres- 

 sion, and are derived from yellow, red, or blue, and in some in- 

 stances from the primitive green, as in the involucre of Cornus. As 

 a whole they present no advance in specialization over yellow flow- 

 ers, and are often smaller and less conspicuous. When the petals 

 of blossoms containing yellow, red, or blue pigments are placed in 

 concentrated alcohol they turn to white. To produce these pig- 

 ments is evidently more or less a tax upon the energies of the plant, 

 which, whenever possible, is avoided. They are not present in the 

 embryonic buds, and may not develop until they are well advanced 



